Method of making dense detergent granules

ABSTRACT

Dense, free-flowing composite detergent additive granules are prepared by contacting hot inorganic salts such as sodium tripolyphosphate with aqueous alkali metal nitrilotriacetate under controlled conditions.

United States Patent [191 Shen et al.

[451 Aug. 26, 1975 METHOD OF MAKING DENSE DETERGENT GRANULES Inventors: Chung Y. Shen; Clayton F. Callis,

both of St Louis, Mo.

Assignee: Monsanto Company, St. Louis, Mo.

Filed: Aug. 24, 1973 Appl. No.: 391,367

References Cited UNITED STATES PATENTS I l/l97l Corliss et 252/385 3,639,289 2/1972 Heymer et a]. 252/527 X Primary Examiner-Benjamin R. Padgett Assistant Examiner-E. Suzanne Parr Attorney, Agent, or Firm-T. N. Wallin; J. E. Maurer; N. E. Willis [57] ABSTRACT Dense, free-flowing composite detergent additive granules are prepared by contacting hot inorganic salts such as sodium tripolyphosphate with aqueous alkali metal nitrilotriacetate under controlled conditions.

6 Claims, No Drawings METHOD OF MAKING DENSE DETERGENT GRANULES BACKGROUND OF THE INVENTION This invention relates to a novel method of making composite detergent additive granules containing alkali metal nitrilotriacetate and an inorganic salt.

It is generally recognized that alkali metal nitrilotriacetates in combination with other inorganic salts such as sodium tripolyphosphate (hereinafter referred to as STP) exhibit excellent performance as detergency builders.

It is desirable to combine the nitrilotriacetate and inorganic salts in the form of composite granules to avoid the problems of segregation, stratification and nonhomogeniety that are inherent when the constituents are dry blended. in some applications, such as machine dishwashing compositions, it is especially desirable to form very dense (at least 0.8 g/cc bulk density) composite granules in order that an effective quantity of detergent will be present within the limited volume of the detergent compartment of the machine dishwashing apparatus.

Conventional methods of making composite granules fail to provide the desired density and, in many instances, result in degradation of the composite constituents. For example, in conventional spray drying pro cedures, a wet slurry of the detergent components is sprayed into a stream of hot gases. This method, however, in volatilizing water from the particles formed, yields low density granules. in the case of STP- nitrilotriacetate spray-dried mixtures, the resultant composite bulk density is normally no more than 0.5 g/cc, which falls short of the 0.8 g/cc minimum density generally required for machine dishwashing applications. In addition, spray drying procedures yield an undesirably high level of decomposed alkali metal nitrilotriacetate, and sodium tripolyphosphate.

Another conventional method of preparing these dense composite granules is to dampen a mixture of the constituents to cause agglomeration and heat to a temperature of above 100C to dry the granules formed. Although this procedure produces somewhat denser granules, the bulk density is still below the 0.8 g/cc required. The process is further disadvantageous if STP is present because it then results in hydrolytic degradation of the STP to less polymerized forms of phosphate.

Accordingly, the need for a method of making dense composite granules without excessive degradation of the constituents is well recognized by those skilled in the detergent art.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of making dense composite granules without excessive degradation of the ingredients thereof.

Basically. these objects are fulfilled by adding aqueous alkali metal nitrilotriacetate onto a hot agitated bed of inorganic salt under critically controlled conditions as hereinafter explained. The invention will be better understood from the following description of the preferred embodiments.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS The process of this invention comprises adding aque ous alkali metal nitrilotriacetate to a hot agitated bed of inorganic salt.

The aqueous nitrilotriacetate can be an aqueous solution or slurry of disodium or trisodium, nitrilotriacetate, or mixtures thereof. The use of trisodium nitrilotriacetate is especially preferred in view of its availability and excellent detergency builder characteristics. The concentration of the nitrilotriacetate in the aqueous mixture should be from 40 to by weight in order to provide a final product containing the amount of nitrilotriacetate desired for various detergency applications.

The inorganic salt to which the nitrilotriacetate is added can be sodium meta silicate, sodium carbonate, sodium sulfate, or mixtures thereof, sodium tripolyphosphate, or mixtures of sodium tripolyphosphate and sodium sulfate.

in the practice of the process the inorganic salt bed is heated to a temperature of from about 300C to about 550C prior to addition of the aqueous nitrilotriacetate. It is critical that this initial temperature be controlled within the above prescribed range because use of initial bed temperatures below about 300C will not permit optimum rates of volatilization of water, and because use of temperatures above 550C will cause undesirably excessive degradation of the nitrilotriacetate. The bed is agitated to effect uniform and homogenous mixing of its contents. Agitation of the bed can be effected by conventional stirring, shaking, vibrating or similar apparatus.

in a particularly preferred embodiment of this invention, STP is fed to the bed directly from a calciner. Since STP is conventionally manufactured in a calcining operation at temperatures of from 400500C, the STP can be directly employed in the present invention without a costly heating or cooling step. Accordingly, this embodiment affords the advantage of significant cost savings.

Sufficient aqueous nitrilotriacetate is added to the bed of salt to provide a composite containing from about 3% to about 20% of the nitrilotriacetate. The aqueous nitrilotriacetate is preferrably heated to a temperature of between C and l 10C, prior to addition to the bed to facilitate rapid vaporization. However, the initial temperature of the aqueous nitrilotriacetate can be as low as 0C or as high as l20C if desired.

In adding the aqueous nitrilotriacetate to the inorganic salt two limitations must be observed. With respect to the following discussion of these limitations, the bed temperatures refer to the average temperature of the mass of inorganic salt to which aqueous nitrilotriacetate is being added.

First, the temperature of the composite bed to which the nitrilotriacetate is added must be below 300C before the bed is permitted to dry. That is, when adding aqueous nitrilotriacetate to a bed having an ititial temperature at or above 300C, sufficient aqueous nitrilo triacetate must be added to lower the temperature below 300C prior to vaporization of all the water added. Evolution of water from the bed will, therefore, be continuous during the period when aqueous nitrilotriacetate is added at temperatures above 300C. If the bed is allowed to become totally dry (as evidenced by no evolution of steam) at temperatures above 300C, excessive decomposition of the nitrilotriacetate will result.

Secondly, addition of the aqueous nitrilotriacetatc nd vaporization of the water contained therein must e complete (particle moisture content less than 1% by 'eight) before the bed temperature is below 160C 'hen STP is present or below 120C when STP is not resent. That is, the bed temperature, at which the evo ltion of steam from the bed ceases, must be above 60C or 120C. if, in the presence of STP, the addition continued or steam is being emitted from any section f the bed after that section has cooled to below 160C, ien the water in the presence of the STP will cause byrolytic reversion of the STP to undesirable phosphates Na P o H O Na HP O NaHPO Note that 'hen STP is not present in the bed, temperatures as )w as 120C can be used since hydrolytic degradation resent no problem. A temperature of at least 120C is :quired to effectively dry the constituents.

The following specific examples are given in order to irther explain and illustrate this invention. However, my are not intended to limit the scope in any way. In 1e examples, all parts and percentages are by weight nless otherwise indicated.

EXAMPLE I Powdered sodium tripolyphosphate (analysis: 92% 'ipolyphosphate; 7% pyrophosphate; 1% trimetaphoshate) heated to 500C is discharged into a screw coneyor to form an agitated bed.

An aqueous slurry containing 45% nitrilotriacetate is eated to 100C and sprayed onto the bed continuously 3r about five minutes. This results in addition of 28.2 arts solution per 100 parts sodium tripolyphosphate. )uring addition of the solution to the bed. steam evoluion is continuous indicating that the bed is not dehyrated at any time during solution addition. The bed :mperature upon completion of solution addition and lehydration (particle moisture content less than 0.5%) 200C.

This procedure produces a granular composite prodct having particle sizes between and 100 US. 'tandard Mesh and a bulk density of about 0.92 ms/cc. The particles contain 11.21 trisodium nirilotriacetate corresponding to the amount of nitriloriacetate added and indicating substantially no degralation thereof. The analysis of sodium tripoly phos- -hate in the product particles is substantially indentical o the analysis of the sodium tripolyphosphate added ndicating absence of hydrolytic degradation in the proess.

EXAMPLE II The procedure of Example 1 is repeated except that additional aqueous nitrilotriacetate is added until the bed temperature is 150C. The assay of sodium tripolyphosphate in the product particles is only with correspondingly higher percentages of pyroand orthophosphates indicating that substantial hydrolytic degradation takes place when the limitations of the invention, as previously discussed, are not observed.

EXAMPLE Ill The procedure of Example 1 is repeated except that less aqueous nitrilotriacetate is added and, upon completion of addition and bed dehydration, bed temperature is 325C.

Product analysis indicates substantial degradation of the nitrilotriacetate added.

What is claimed is:

l. A method of making composite detergent additive granules containing from 3% to 20% alkali metal nitrilotriacetate the balance of said granules being an inorganic salt selected from the group consisting of so dium metasilicate. sodium carbonate, sodium sulfate, mixtures thereof, STP and mixtures of STP and sodium sulfate, said method comprising adding aqueous alkali metal nitrilotriacetate, containing from about 40% to about 70% by weight alkali metal nitrilotriacetate and having a temperature of from about 0C to about C, to an agitated bed of the inorganic salt, having an initial temperature of from about 300C to about 550C; sufficient aqueous nitrilotriacetate being added to cool the bed below about 300C prior to dehydration of the bed and the addition of aqueous nitrilotriacetate and dehydration of the bed being complete at a bed temperature of at least about C when STP is present or about 120C when STP is not present.

2. The process of claim 1 wherein the aqueous alkali metal nitrilotriacetate has a temperature of 80 to 3. The process of claim l wherein the inorganic salt has a temperature of from about 400C to about 500C.

4. The process of claim 1 wherein the aqueous alkali metal nitrilotriacetate contains 40 to 50% nitrilotriacetate.

5. The process of claim 1 wherein the inorganic salt is sodium tripolyphosphate.

6. The process of claim 1 wherein the alkali metal nitrilotriacetate is trisodium nitrilotriacetate.

* k i i 

1. A METHOD OF MAKING COMPOSITE DETERGENT ADDITIVE GRANULES CONTAINING FROM 3% TO 20% ALKALI METAL NITRILOTRIACETATE THE BALANCE OF SAID GRANULES BEING AN INORGANIC SALT SELECTED FROM THE GROUP CONSISTING OF SODIUM METASILICATE, SODIUM CARBONATE, SODIUM SULFATE, MIXTURES THEREOF, STP AND MIXTURES OF STP AND SODIUM SULFATE, SAID METHOD COMPRISING ADDING AQUEOUS ALKALI METAL NITRILOTRIACETATE, CONTAINING FROM ABOUT 40% TO ABOUT 70% BY WEIGHT ALKALI METAL NITRILOTRIACETATE AND HAVING A TEMPERATURE OF FROM ABOUT 0*C TO ABOUT 120*C, TO AN AGITATED BED OF THE INORGANIC SALT, HAVING AN INTIAL TEMPERATURE OF FROM ABOUT 300*C TO ABOUT 550*C, SUFFICIENT AQUEOUS NITRILOTRIACETATE BEINT ADDED TO COOL THE BED BELOW ABOUT 300*C PRIOR TO DEHYDRATION OF THE BED AN THE ADDITION OF AQUEOUS NITRILOTRIACETATE AND DEHYDRATION OF THE BED BEING COMPLETE AT A BED TEMPERATURE OF AT LEAST ABOUT 160*C WHEN STP IS PRESENT OR ABOUT 120*C WHEN STP IS NOT PRESENT.
 2. The process of claim 1 wherein the aqueous alkali metal nitrilotriacetate has a temperature of 80* to 110*C.
 3. The process of claim 1 wherein the inorganic salt has a temperature of from about 400*C to about 500*C.
 4. The process of claim 1 wherein the aqueous alkali metal nitrilotriacetate contains 40 to 50% nitrilotriacetate.
 5. The process of claim 1 wherein the inorganic salt is sodium tripolyphosphate.
 6. The process of claim 1 wherein the alkali metal nitrilotriacetate is trisodium nitrilotriacetate. 